Neither do I know what's your "best email address" nor is there a "Mozilla account". Thunderbird is an application run on your device, so it does not provide email addresses, nor the service to transmit emails. But Thunderbird can be used with your existing email address and service provider to send, receive, sort and search your email messages.

What are you trying to achieve in the first place?

I've got hundreds of contacts in the Zoho program's "Address Book" and I can't find a similar thing in gmail

I'm not exatcly sure what you're looking for. Google has Google Contacts, which is web based, so you can manage your contacts via a web browser. https://contacts.google.com/

On top of that you could create an address book in Thunderbird linked to your Google Contacts.

after many hours of searching online to try to figure out how to export the address book to gmail and finally learning it can't be done, I'm hoping to learn how to add the gmail program to Thunderbird.

I have no idea about Zoho, so I can't help with exporting your contacts from there. Alternatively you could create the Google Contacts address book in Thunderbird and manually enter your contacts in Thunderbird. They will then be synced automatically with your Google Contacts. Entering contacts manually isn't a fun job, and I don't know how many cintacts you have. So I'm not sure if this would be an option at all.

I'm going to turn 77 in a few months. Maybe I should give up all this tech stuff.

You certainly should not give up.